This invention relates to an optical transmission system comprising an optical path cross-connect device (OPXC) and an electrical cross-connect device (EXC). More particularly, the invention relates to an optical transmission system in which transmission lines can be switched without momentary disconnect.
Transmission systems using optical technology are expected to find use in the future as broad-band transmission systems as information is dealt with more rapidly and in greater quantities.
FIG. 46 is a diagram for describing an optical network, and FIG. 47 is a diagram showing the connections between an optical path cross-connect device (OPXC) and an electrical cross-connect device (EXC). The network includes optical path cross-connect devices (OPXC) 51, optical transmission lines 52 which transmit optical signals of wavelength .lambda..sub.0, electrical cross-connect devices (EXC) 53, switches (SW) 54 and transmission lines 55 which transmit electrical or optical signals. Each optical path cross-connect device 51 accommodates a plurality of working optical transmission lines (k1), a plurality of standby optical transmission lines (k2) and a plurality of interface links (k3) which interface the electrical cross-connect device 53. The electrical cross-connect devices 53 and switches 54 perform switching in units of virtual paths VP and virtual channels VC, respectively.
Each optical cross-connect device 51 usually outputs optical signals upon switching over the outgoing lines of the optical signals that have entered from the optical transmission lines and interface links and branches optical transmission signals, which are addressed to its own node, to the electrical cross-connect device 53.
In such an optical network, there is a requirement for a function in which the path is switched over to the standby system in response to a transmission line fault, after which the path is switched back to the working system, without momentary disconnect, in response to fault recovery, and for a function in which the working path is switched over, without momentary disconnect, in dependence upon traffic. Further, this function for uninterrupted switching of transmission lines is required from the viewpoint of network maintenance and operational administration. The function for switching transmission lines without momentary disconnect has already been implemented in electrical cross-connect devices. Naturally, it is necessary to so arrange it that transmission lines can be switched without momentary disconnect, in the same manner as in electrical cross-connect devices, also in optical cross-connect devices which perform network routing of optical signals from the electrical cross-connect devices.
FIG. 48 is a diagram showing the architecture of a conventional optical path cross-connect device which takes into account the switching of transmission lines without momentary disconnect in an optical path. Shown in FIG. 48 are the optical path cross-connect device (OPXC) 51 and the electrical cross-connect device (EC) 53. The optical cross-connect device 51 accommodates working optical transmission lines 52a.sub.1, 52a.sub.2 and standby optical transmission lines 52b.sub.1, 52b.sub.2, as well as working interface links 55, 56 for interfacing the electrical cross-connect device 53. The optical path cross-connect device 51 has an optical space switch 51a and a number of optical phase adjusting buffers 51b. The electrical cross-connect device 53 has an electrooptic converting function for converting an electric signal to an optical signal and sending the optical signal to the working interface link 55, and an optoelectric converting function for converting an optical signal, which enters from the optical path cross-connect device 51 via the interface link 56, to an electric signal and entering the electric signal into an ATM switch (not shown).
FIG. 49 is a diagram for describing the operation of transmission line switching without momentary disconnect according to the prior art. FIG. 49 illustrates a case where two units OA1 and OA2 having the architecture illustrated in FIG. 48 are provided and transmit data to each other. The first unit OA1, which comprises an electrical cross-connect device EXC1 and an optical path cross-connect device OPXC1, and the second unit OA2, which comprises an electrical cross-connect device EXC2 and an optical path cross-connect device OPXC2, are interconnected by a working optical transmission line 52a and a standby optical transmission line 52b.
Each of the optical path cross-connect devices OPXC1, OPXC2 routes an optical signal that has entered from an optical transmission line to a desired output transmission line or to the electrical cross-connect devices EXC1, EXC2 and routes an optical signal from the electrical cross-connect devices EXC1, EXC2 to a desired output transmission line. For example, an input signal that has entered from the electrical cross-connect device EXC1 of the first unit OA1 via the input interface link 55 is branched in two directions by the optical path cross-connect device OPXC1 and outputted to the working optical transmission line 52a and standby optical transmission line 52b to reach the second unit OA2. The optical path cross-connect device OPXC2 of the second unit OA2 routes the optical signal that has entered from the working optical transmission line 52a to the output interface link 56 leading to the electrical cross-connect device EXC2. Thus, the optical signal from the electrical cross-connect device EXC1 of the first unit OA1 is transmitted to the electrical cross-connect device EXC2 of the second unit OA2.
If a signal flowing on the working transmission line 52a is switched over to the standby transmission line 52b under these conditions, the optical path cross-connect device OPXC2 implements the working/standby changeover and routes the optical signal that has entered from the standby optical transmission line 52b to the output interface link 52b leading to the electrical cross-connect device EXC2. As a result, the switching of the transmission lines is performed without a momentary disconnect and the signal from the electrical cross-connect device EXC1 is transmitted to the electrical cross-connect device EXC2 without interruption.
It is necessary that the optical path length between the two systems before switching between working/standby be made equal to the optical path length between the two systems after switching between working/standby. To achieve this, the practice in the prior art is to adjust phase by the optical phase adjusting buffers 51b, which are provided in the optical path cross-connect device OPXC2, in such a manner that the optical paths are equalized, and switch between working/standby synchronously in the sending and receiving optical path cross-connect devices. With this conventional method of switch optical transmission lines, however, it is required that the optical signal phase adjustment be performed in the form of light in the optical path cross-connect device OPXC, and that the optical signal be switched at a high speed but to a degree that will not produce a bit error. This is very difficult to realize.